<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .a1{
            width: 200px;
            height: 200px;
            background: #666;

            /* border边框 */
            /* border-top: 1px solid red;
            border-right: 2px dashed blue;
            border-bottom: 4px dotted green; */
            border: 4px solid red;
            /* border-bottom: 1px solid #fff; */

            /* padding内边距 */
            /* padding-left: 25px;
            padding-top: 20px;
            padding-right: 50px; */
            /* padding: 20px; */
            padding: 20px 30px;

            /* margin外边距 */
            /* margin-bottom: 3px;
            margin-left: 20px; */
            /* margin: 50px; */

            /* 盒子的水平居中 */
            margin: 50px auto;
            /* 文本的水平居中 */
            text-align: center;
        }
        .a2{
            width: 200px;
            height: 200px;
            background: #999;
        }
    </style>
</head>
<body>
    <div class="a1">aaaaa</div>
    <div class="a2"></div>
</body>
</html>